1. Field of the Invention
The present invention relates to a voltage applying apparatus for driving a light signal emitter. More specifically, the present invention relates to a voltage applying apparatus for driving an optical shutter array.
2. Description of the Related Art
An optical shutter array is used in some image forming apparatuses such as a copying machine or a printer. The optical shutter array includes optical shutter elements each of which is a small pixel unit having an electro-optical effect, and used for exposing a photoreceptor. As light to the photoreceptor is intercepted/passed by the optical shutter element, an image is formed on the photoreceptor. By controlling magnitude of voltage driving the optical shutter element, the amount of exposure of the photoreceptor is adjusted, whereby a halftone image can be represented. An image is formed on a recording sheet based on the image formed on the photoreceptor.
However, the amount of light transmitted through the optical shutter element has temperature dependency. Therefore, when the temperature of the optical shutter element differs, the transmitted light amount differs, even when the driving voltage is kept constant.
FIG. 7 shows temperature dependency of the optical shutter element. Provided that the temperature of the optical shutter element is constant, the transmitted light amount gradually increases as the driving voltage increases, reaches a maximum amount at a certain driving voltage, and thereafter the amount decreases. When curves showing the transmitted light amount corresponding to the driving voltage at various temperature are plotted, it can be understood that the higher the temperature, the larger the driving voltage which maximizes the transmitted light amount (hereinafter, the driving voltage which maximizes the transmitted light amount will be referred to as optimal driving voltage).
A heat generating portion in an image forming apparatus changes ambient temperature, and varies temperature of the optical shutter element. The amount of light transmitted through the optical shutter element varies with the temperature variation, resulting in variation in density of the recorded image. Thus, image quality is considerably deteriorated because of the variation in ambient temperature.
In order to prevent variation in image density caused by temperature variation of the optical shutter element, a technique for detecting temperature of the optical shutter element and changing driving voltage accordingly is disclosed in Japanese Patent Laying-Open No. 63-189269. In the image forming apparatus disclosed in Japanese Patent Laying-open No. 63-189269, a temperature detecting element detects temperature of PLZT optical shutter array and outputs a signal indicative the detected temperature. The signal is subjected to A/D conversion, and based on the A/D converted signal and prescribed data corresponding to the temperature in a ROM, driving voltage for the PLZT optical shutter array is controlled.
However, the optimal driving voltage for the PLZT optical shutter array varies widely, and the optimal driving voltage is different in different PLZT optical shutter array used in image forming apparatus.
FIG. 8 shows variation of optimal driving voltages for two optical shutter arrays. In the graph, the dotted line 201 represents the optimal driving voltage with respect to temperature for a certain optical shutter array, while dotted line 202 represents the optimal driving voltage with respect to temperature for another optical shutter array. The change rate of the optimal driving voltage with respect to time is almost the same in two optical shutter arrays and is about 0.27 V/.degree. C. The solid line 203 represents an average optimal driving voltage for the two optical shutter arrays.
The variation of the optimal driving voltage results from variation in the processed shape of the optical shutter array, and it is difficult to eliminate the variation in processed shape. In order to cope with the problem of variation of the optimal driving voltage, it is necessary to control the driving voltage in accordance with temperature characteristic of individual optical shutter array in each image forming apparatus employing the optical shutter array.
Such variation of individual optical shutter array is not addressed in the image forming apparatus disclosed in the aforementioned Japanese Patent Laying-Open No. 63-189269. Therefore, it is necessary to rewrite prescribed data in the ROM for controlling the driving voltage of the optical shutter array which corresponds to the signal from the temperature detecting element, for each image forming apparatus having optical shutter array of different temperature characteristic.
However, rewriting of data in the ROM individually for each image forming apparatus increases manufacturing cost. Further, in the above described image forming apparatus, the signal indicative of the temperature has to be A/D converted, which leads to complicated circuit structure.